fix(handbook): stop calling resolve('') in root-landing check

[nerdalytics]

+layout.svelte computed isRootLanding with a second clause page.url.pathname === resolveHref(''). resolveHref forwards to $app/paths resolve(), which throws on any argument not starting with /. The validation is present in @sveltejs/kit 2.65.0 and 2.70.2; the earlier pinned 2.55.0 had none (resolve('') returned base + '/'). The lockfile first picked up a validating kit in b677828 (#149).

Consequences, reproduced locally with BASE_PATH=/beacon npm run build on kit 2.70.2:

• Prerendering 500s on every page except the root redirect stub ([500] GET /beacon/latest ... Cannot use resolve(...) in the build log). prerender.handleHttpError: 'warn' in handbook/svelte.config.js downgraded those to warnings, so CI was green while the Pages artifact contained only index.html, 404.html, the redirect stubs and the .md/llms.txt endpoints (4 HTML files total). The currently published site, from the Sep 11 workflow_dispatch run, is in that state: /beacon/latest/ and /beacon/v2000.0.0/introduction return 404.
• GitHub Pages serves the fallback 404.html with a 404 status for any missing route; the client app boots on it and the same resolve('') throws uncaught during render, leaving a blank page with Cannot use resolve(...) (got "") in the console.

The clause adds nothing even where it does not throw: in the client and during prerendering, resolve('/') already equals base + '/', the only value the second clause could ever match. This removes it.

The PR also contains:

• DocLayout derived its version prefix by stripping base from page.url.pathname. During prerendering $app/paths base is relative (overridden per request), so the prefix came out empty and VersionLink emitted root-level hrefs — ../lens from /latest/select, ../migration from /v1000.0.0/introduction — the two 404s the crawl warned about. It now derives from page.route.id, which carries no base on server or client; the prev/next index comparison had the same mismatch and is fixed the same way.
• 14 hardcoded absolute markdown links (](/latest/hooks-overview) and siblings) lacked base entirely: they 404 outside /beacon for users and fail the crawl's base check. They now go through VersionLink.
• prerender.handleHttpError: 'warn' is removed, restoring the kit default 'fail': any non-OK response during prerendering, including 500s from render crashes, fails the build instead of shipping a hollow artifact green.

Verification, same build command, after all changes: exit 0, zero 404/500 lines during prerendering, 232 HTML files including latest.html and v2000.0.0/introduction.html; static-HTML anchors resolve inside base (e.g. ../latest/hooks-overview from /beacon/latest/state).